The present invention relates to a hydraulic vehicle braking system.
Known from published patent documents DE 196 36 432 A1 and WO 98/31576 are hydraulic vehicle braking systems having wheel brakes for four wheels distributed to a first and a second vehicle axle; a hydraulic external energy source; electrically controllable service brake valve configurations situated between the external energy source and the wheel brakes; a braking value indicator operable by a brake pedal for performing service braking via hydraulic external energy using the service brake valve configurations; a dual-circuit master brake cylinder that is operable via the brake pedal and has a first master brake cylinder chamber and a second master brake cylinder chamber for performing auxiliary braking via muscle-powered energy, a normally open operation mode switching valve that is controllable for the service braking operation in a closed position being situated in each brake circuit between the master brake cylinder and the wheel brakes being able to be supplied with pressure medium by this master brake cylinder, and at least the service brake valve configurations being controllable via a control and regulating device. In this context, the wheel brakes of a front vehicle axle belong to a first brake circuit, and the wheel brakes of a back vehicle axle are assigned to the second brake circuit. In the case of a power outage and a thus inoperable control and regulating device, the electrically controllable service brake valve configurations are not able to be put into operation, and the four wheel brakes of the two vehicle axles are connected through the normally open operation mode switching valves to the dual-circuit master brake cylinder. As a result, all four wheel brakes and, consequently, all wheels of the vehicle are able to be braked by muscle force on the brake pedal. In this context, starting from a vehicle to be equipped and from load examples to be assumed for this vehicle, one skilled in the art may determine a distribution of the braking force to the front wheel brakes and the back wheel brakes by selecting the dimensions of the brake components. The control and regulating device may be set up, for example, according to the SAE Technical Papers Series No. 960991 entitled xe2x80x9cElectrohydraulic Brake Systemxe2x80x94The First Approach to a Brake-by-Wire-Technology,xe2x80x9d page 111, under the heading xe2x80x9cSafety Concept,xe2x80x9d such that, in response to the failure of a service brake valve configuration, the control and regulating device determines whether using external energy for three wheels results in a shorter braking path or using muscle-powered energy for four wheels results in a shorter braking path in order after such a determination to control those valves resulting in the shorter braking path or to leave them alone. In this context it should be mentioned that on icy or snowy roads, for example, it may be better to brake four wheels using muscle-powered energy, thereby making the vehicle more easily controllable than in the case of asymmetrical braking using only three wheel brakes and operation via external energy. It cannot be completely ruled out that in the case of operation via external energy, air or gas dissolved in the hydraulic pressure medium results in the formation of air or gas bubbles due to a pressure drop in the hydraulic pressure medium. Air or gas bubbles in the hydraulic pressure medium make it compressible so that in the case of a sudden transition from the service braking operation via external energy to the auxiliary braking operation via muscle-powered energy, the disadvantage may arise that the air or gas bubbles are to be compressed via muscle-powered energy using the master brake cylinder. Since the master brake cylinder has a limited pump capacity for each brake pedal travel, it cannot be ruled out that as a result of the elasticity of the air or gas bubbles, the wheel brake pressure does not increase as much as desired for decelerating the vehicle.
A hydraulic vehicle braking system known from published patent document WO 98/28174 has wheel brakes for four wheels distributed to a first and a second vehicle axle; a hydraulic external energy source; electrically controllable service brake valve configurations situated between the external energy source and the wheel brakes; a braking value indicator operable by a brake pedal for performing service braking via hydraulic external energy using the service brake valve configurations; and a dual-circuit master brake cylinder operable by the brake pedal for performing auxiliary braking using muscle-powered energy, where a normally open operation mode switching valve that is able to be controlled for the service braking operation in a closed position is situated in each brake circuit between the master brake cylinder and the wheel brakes able to be supplied with pressure medium by this master brake cylinder; at least the service brake valve configurations are controllable by a control and regulating device; a cylinder-piston configuration is installed in each case between the wheel brakes of those vehicle axles designed for a greater contribution to the vehicle braking and the corresponding electrically controllable service brake valve configurations; and the wheel brakes designed for a greater contribution to the vehicle braking are connected to the operation mode switching valves associated with the two brake circuits of the master brake cylinder, has the advantage that the wheel brakes of this vehicle axle and the dual-circuit master brake cylinder are hydraulically isolated from the pressure medium that is able to be placed under pressure by a pump for the service braking operation and is able to be stored in pressure accumulator and, therefore, may contain air and/or gas in the form of bubbles, for example due to a pressure drop. In this respect, the combination of master brake cylinder and the two corresponding wheel brakes isolated from the pressure medium able to be pumped by the pump has the same degree of reliability as a conventional hydraulic vehicle braking system. Since only the wheel brakes of one vehicle axle are able to be braked via the pressure medium from the master brake cylinder, the vehicle axle that is typically the more loaded vehicle axle is selected for braking using the pressure medium from the master brake cylinder.
U.S. Pat. No. 5,941,608 discloses another hydraulic vehicle braking system that is set up, on the one hand, for braking four vehicle wheels via external energy using service brake valve configurations and, on the other hand, for braking via muscle-powered energy using a dual-circuit master brake cylinder. In this context, four cylinder-piston configurations are provided for isolating the four wheel brakes from the pressure medium that is able to be placed under pressure by a pump and is able to be controlled by the service brake valve configurations to direct against the individual pistons of the cylinder-piston configuration. As a result, braking pressure is able to be reliably generated in the four wheel brakes via the master brake cylinder and via the pressure medium contained therein. The necessary technical outlay for this is noticeably higher than for a hydraulic braking system according to U.S. Pat. No. 5,941,608.
The hydraulic vehicle braking system of the present invention is less expensive than the hydraulic vehicle braking system according to U.S. Pat. No. 5,941,608, and provided that no gas and/or air bubbles are present in the vehicle braking system, renders it nevertheless possible to brake via the wheel brakes of the vehicle axle contributing less to the vehicle deceleration, so that improved vehicle deceleration with typically better controllability of the vehicle is achieved in comparison to the hydraulic vehicle braking system according to document WO 98/28174.
One advantage of this invention is that, as a result of an individual operation mode switching valve being present for each wheel brake cylinder of a brake circuit, only one wheel brake is isolated from the master brake cylinder in the case of a technical malfunction, for example, and no longer contributes to braking. In addition, the same advantage may be provided for the vehicle axle whose wheel brakes contribute less to the total vehicle deceleration in the normal case. Another feature of the present invention results in the possibility of supplying two wheel brakes of a vehicle axle with pressure medium from the master brake cylinder via only one operation mode switching valve.